Tuesday, April 12, 2011

Risk homeostasis, Munich Taxi-cabs and the Nanny State

There is an odd coexistence between two conflicting safety policies that may well be pursued by the same accident prevention agency. The first seeks to improve safety by alleviating the consequences of risky behaviour. It may take the form of seat belt installation and wearing, airbags, crashworthy vehicle design, or forgiving roads (collapsible lamp posts and barriers). This policy offers forgiveness for a moment of inattention or carelessness. The second policy seeks to improve safety by making the consequences of imprudent behaviour more severe and includes things such as speed bumps, narrow street passages, and fines for violations. Here, people are threatened into adopting a safe behaviour; a moment of inattention or carelessness may have a dire outcome. 

While these two policies seem logically contradictory, neither is likely to reduce the injury rate, because people adapt their behaviour to changes in environmental conditions. Both theory and data indicate that safety and lifestyle dependent health is unlikely to improve unless the amount of risk people are willing to take is reduced. (here - my emphasis) 

The above passage points out a common logical absurdity, and contains an important lesson for Australian’s with and overeager obsession of controlling personal choices through ‘nanny state’ regulations. More on the nanny state a little later. 

First, it is important to examine the hypothesis of risk homeostasis to properly understand the implication of the opening quote, since it claims that neither of the two contradictory policies aimed towards improving safety are effective. 

The essential argument of risk homeostasis is that humans have an inbuilt level or risk that they gravitate towards in response to their external environment. If we reduce the risk of an activity, people will compensate by finding other risky activities as a replacement, or undertaking the activity in a more extreme manner. For example, if we ban smoking tobacco, which doesn’t seem like such a remote possibility, do we really expect smokers to replace their habit with fruit snacks and yoga? Or might they compensate by increasing their alcohol consumption or perhaps smoking dope instead. 

Risk homeostasis is not to be confused with risk compensation, which suggests that individuals will behave less cautiously in situations where they feel "safer" or more protected, but that we don’t necessary return to a predetermined risk equilibrium point. 

Improving transport safety is an area where there is strong evidence risk compensation, and indeed of risk homeostasis. 
One reason I am against mandatory bicycle helmets is due to the increased dangers posed by this perception of safety. Not only do cyclists tend to behave more aggressively, but helmet wearing changes the behaviour of other road users. It decreases the rate of cycling, and the effect of fewer cyclists on the road is to increase the danger of the remaining cyclist, but it also makes vehicle drivers act more carelessly around cyclists. 

This study fitted a number of bicycles with video cameras and ultrasonic sensors to detect the proximity of vehicles as they passed cyclists on the road. They found that vehicles passed helmeted riders about 8.5cm closer than cyclists without helmets. The suggestion is that drivers perceive there is less risk from clipping a helmeted rider and that helmeted riders are more experienced and unlikely to ride erratically. They also found driver give female cyclists much more room! 

Of course this behaviour is all based on perceptions. Helmets themselves provide minimal protection in a limited number of head collisions, and can exacerbate brain injury for some other types of collisions where helmets can increase rotational acceleration during a collision. 

The best experimental evidence of risk homeostasis is the famous Munich Taxi-cab experiment, where for three years half the taxi cabs in a fleet had ABS brakes and the other half didn’t, and various monitoring and testing took place including fitting all vehicles with accelerometers. 

Among a total of 747 accidents incurred by the company's taxis during that period, the involvement rate of the ABS vehicles was not lower, but slightly higher, although not significantly so in a statistical sense. These vehicles were somewhat under-represented in the sub-category of accidents in which the cab driver was judged to be culpable, but clearly over-represented in accidents in which the driver was not at fault. Accident severity was independent of the presence or absence of ABS.
Subsequent analysis of the rating scales showed that drivers of cabs with ABS made sharper turns in curves, were less accurate in their lane-holding behaviour, proceeded at a shorter forward sight distance, made more poorly adjusted merging manoeuvres and created more "traffic conflicts". This is a technical term for a situation in which one or more traffic participants have to take swift action to avoid a collision with another road user. Finally, as compared with the non-ABS cabs, the ABS cabs were driven faster at one of the four measuring points along the route. All these differences were significant. 

To put this experiment in the context of our original two options for reducing risk, ABS brakes are an example of an action that reduces the consequences of risky behaviour. Hence such actions decrease total risk. 

But the study did not end there, and finds some evidence that the opposite type of strategy, increasing the consequences of risk taking, has quite an effect. 

In a further extension of their study, the researchers analysed the accidents recorded by the same taxi company during an additional year. No difference in accident or severity rate between ABS and non-ABS vehicles was observed, but ABS taxis had more accidents under slippery driving conditions than the comparison vehicles. A major drop, however, in the overall accident rate occurred in the fourth year as compared with the earlier three-year period. The researchers attributed this to the fact that the taxi company, in an effort to reduce the accident rate, had made the drivers responsible for paying part of the costs of vehicle repairs, and threatened them with dismissal if they accumulated a particularly bad accident record. 

My favourite example of different effects of increasing consequences of risk taking versus decreasing the consequences is here

Sometimes, students would deny that they drive more recklessly when wearing a seatbelt. Tullock liked to illustrate the idea of offsetting behaviour for them by asking what they'd do if a large spike extended from the steering wheel and pointed directly at their heart. Wearing a seatbelt is a mild form of that effect, but in reverse. Tullock's students came to call the thing the "Tullock Spike"

Australian policy makers could learn some lessons from risk homeostasis. For any returning Aussie from time abroad the degree of over-regulation can be a shock. One friend recently returned from three years in Paris and said that it was the one thing that enraged them the most about coming home. 

Think about it. We can’t buy alcohol at the supermarket, nor drink it in a public place, nor smoke in a building even if the owner is trying to run a cigar smoking cafe. In fact the body corporate of an apartment building recently passed a by-law to stop people smoking in their own homes! 

With the recent surge in anti-smoking opinion, not only will smokers pay ridiculous taxes, cigarette producers will need to adorn their prized products with pictures of diseased organs, but not their own brand label. Luckily, alcohol is exempt from such measures which might make the Chinese communist party blush, yet arguable alcohol is a far greater public health concern. 

We can’t buy fireworks or ride without a helmet. There are now calls to ban topless bathing on NSW beaches and there is the infamous internet filter proposed my communications minister Conroy. Oh, I almost forgot the proposal to ban teachers from using red pens when marking because red is an aggressive colour!

We can’t be expected to navigate construction zones on the street without 17 different warning signs, stop-go lollipop ladies, flashing lights and orange fencing, nor, it seems, are we expected to navigate over treacherous cracks in footpaths, with councils often paying compensation to ‘victims’ of such treachery. We have the slowest speed limits yet many would say the worst drivers. 

Policy makers must believe the average Australian has both multiple personality disorder and signs of schizophrenia. Someone who can perfectly judge their own financial risk when taking on massive debt, can choose their own career, raise their own children, run their own businesses and judge the associated risks, yet when it comes to the basics of life, like having a beer, tanning your breasts, or navigating the street, we all become complete morons incapable of rational behaviour. 

I have no problems with governments intervening to protect people for the actions of others, but they should be make their own choices about their personal safety. The strongest argument in favour of this position is the theory of risk homeostasis. It seems we really can’t save people from themselves. In fact the parent in me suggests that all this molly-coddling decreases our ability to judge real risks when they arise.  

Rant over.


  1. what if our appetite for risk is dependent on the setting? There's the lemming problem where you're more likely to jump off a cliff into murky water if others are doing it as well. Do you think that's it's possible to have locally non-stationary risk preferences? If so, how does this affect what you've suggested above?

  2. Top post Cameron!
    Strong sympathy with your way of thinking.
    I already heard about the ABS study in a paragliding article by Bruce Goldsmith, and now I refer to risk homeostasis theory often.

  3. So very true - in fact on our trip to Europe I was prepared to be "nannied" everywhere, having heard of the excesses of the EU bureaucracy.

    Compared to Australia, it was outright anarchy! And much much more sensible. (sure there were some exceptions).

    Coming back, you realise how regulated we are - it all seems LCD to me.

    Kind of puts Singapore in a different light.....

  4. Stephen,

    Yes, I don't believe our risk appetite is perfectly fixed over time - it obviously changes as we age and depends a bit on our social setting. For an individual I don't think risk appetites can change much in the short term, and at society level, I don't think it changes much at all.

    Society has become safer, but look what we do now - http://www.videosforall.ubx.cc/video/insane-videos-compilation.php

    Thanks Nick and Prince for the comment. You really do have to get out of the country to realise the situation we put ourselves in.

  5. It's easy to pick a group of nonsense rules and regulations and thereby condemn an entire system. I note by your profile pic that you are fairly young, so you have missed many changes in society that have been fostered by the Nanny.
    When I grew up, it seemed every other week you'd hear of one or two children gone missing only to be discovered days later locked in a discarded refrigerator (ice chest) - in the early days they actually had self-locking mechanisms that prevented escape.
    Were the parents of these children outraged that the Nanny came in and forced manufacturers to change the locking system? Of course not, there are hundreds of people alive today because of the regulation - thousands of lives not ruined by a child's death.
    By your logic there should be as many children's deaths in refrigerators because they are taking greater risks. Odd, I haven't heard of one in about 50 years.
    Today, we have the same problem with parents reversing over children. Your argument would be that the Nanny state shouldn't regulate vehicles be equipped with devices that disable reversing if one is developed that can sense a child playing behind the vehicle. Where is your risk logic in that? Parents will care less for their kids? Kids will run behind the vehicle more frequently? Sorry, using extreme examples to prove a general thesis is unscientific.
    For example, using taxi drivers as a proxy for the general driving public? Please, pull the other one.

  6. Great post cameron.. very true. On the bike I'm starting to notice that a certain class of motorists seem to be far more aggressive to sports cyclists.

    I wonder about the [url=http://www.londoncyclist.co.uk/cycling-london/the-mary-poppins-effect/]Mary Poppins Effect [/url] and how effective it actually is.

  7. Thanks for the comment Anon.

    I’m not sure if my post came across that way, but I am not condemning the entire system. Just highlighting how many invasive regulations are there to save us from ourselves without any consideration of offsetting behaviours.

    Also, I don’t fully agree with the first quote.

    The taxi-cab experiment did show that making drivers financially responsible for their accidents did reduce the accident rate. This contradicts the idea that increasing the consequences of risk taking reduces the occurance. I’m not sure why you have a problem with taxi drivers as experimental subject though.

    The risk of accidental death has decreased amazingly in the past century or so. That is strong evidence against some absolute fixed level risk homeostasis theory.

    But how much of that is due to regulation of safety standards and how much is due to other factors – like increased parental supervision (due to lowered risk tolerance), or improved medical care so that many injuries no longer lead to death – is completely uncertain.

    Further, we have real problems actually measuring risk. We might know that far fewer children drown in the Brisbane River these days, but far fewer children swim in the river. Fewer children get trapped in discarded fridges (I still have one of these locking fridges from my grandfather) because there are both fewer discarded fridges and fewer children playing near discarded appliances unsupervised.

    Lastly, you need to remember that all regulations have a cost. So if the regulation to have non-locking fridge doors meant all fridges cost a little extra. This money, society wide, could have been used for other things that may also have reduced the risk of death – better diets, health care or even other regulations.

    I just want to raise how the benefits of regulation are not clear cut, and when evaluating the cost/benefit there are offsetting behaviours to consider.

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